Various devices for forming fluid cushions or fluid bearings have been proposed for the contactless support of a web as the latter changes directions during its course of travel. These webs may have printed matter on both sides and which must be dried without contact of the web by any supporting rollers or the like. These running webs to be dried must pass through a number of runs of different directions and the web must be held out of contact with anything until the ink has dried. These contactless supporting devices have a generally partially cylindrical surface through which pressurized air is introduced through various slots, holes, or apertures, or other designs or patterns.
Examples of such prior art devices are shown in the Daniels U.S. Pat. No. 2,689,196 which issued Sept. 14, 1954 wherein a series of holes are simply formed in the cylindrical surface for the passage of pressurized air therethrough to support and guide a web passing over the drum. Another example is shown in the U.S. Pat. No. 3,097,971 to Carlisle which issued July 16, 1963 and which has a series of slits in the curved supporting surface and which extend either longitudinally or transversely to the web, or both. Air under pressure is then passed through these slits to form a cushion between the drum and the web. Still another example is shown in the U.S. Pat. No. 3,567,093 which issued Mar. 2, 1971 to Johnson and wherein a series of holes is formed in the drum-like supporting member and a series of arcuate baffle members extend around a portion of the drum and in the direction of web movement. Still other examples of the prior art are shown in U.S. Pat. No. 3,127,080 issued Mar. 31, 1964 to Allander et al and U.S. Pat. No. 3,186,326 which issued June 1, 1965 to Schmidt and in both of the latter patents, air is passed through slots or holes in the cylindrical members to form a pressurized cushion between the cylindrical member and the web.
One shortcoming of the prior art devices, particularly that as shown in the said Carlisle U.S. Pat. No. 3,097,971, is that of lateral tracking instability or side drifting of the web. Furthermore, corrective pressure forces are difficult to obtain because of air slots which extend in the direction of web travel are located beneath the web to form the pressure cushion.
In certain prior art devices, other than the said Carlisle U.S. Pat. No. 3,097,971, a very large air flow rate is required to support a web under high tension with adequate clearance between the web and the support surface. In these other devices, the air in the pneumatic support cushion is relatively free to escape out from under the longitudinal edges of the web.